PROJECT SUMMARY/ABSTRACT Some individuals with normal hearing thresholds (NHTs) demonstrate considerable challenges perceiving speech in the presence of background noise. Evidence from recent studies suggests that this impairment may be the result of cochlear synaptopathy (CS), degeneration of the synapses between cochlear hair cells and the auditory nerve, observed in animal models following exposure to high-intensity noise that does not permanently alter hearing thresholds. Still, it is not known exactly how noise-induced CS would contribute to impairments in speech-in-noise perception in humans with NHTs. CS engenders auditory nerve fiber degeneration and weakens auditory nerve phase-locking to the precise timing information in auditory signals; therefore, the goal of this research is to determine whether temporal processing impairments from noise- induced CS are consistent with speech-in-noise perceptual deficits in listeners with NHTs. This investigation will contrast listeners with high- and low-likelihood of noise-induced CS, as assessed by a measure of summed auditory nerve fiber responses, on two mechanisms for segregating speech streams that depend on accurate coding of precise timing information. Aim #1 of this investigation will examine use of interaural timing difference cues for spatial attention to speech streams and the degree to which attention to a target stream modulates cortical responses. Aim #2 will utilize a novel task to assess participants? ability to use pitch cues that can only be coded by auditory nerve phase-locking to perceptually separate speech streams. Cortical entrainment to the temporal envelope of speech, which has been shown to increase perceptual representation of individual streams, will also be examined during this paradigm. Compared to participants with a low likelihood of CS, it is expected that those with a high likelihood of having CS will perform more poorly on tasks that require use of precise timing cues to segregate speech sounds from competing streams. These listeners should also demonstrate lower cortical modulation and neural synchrony to speech streams, demonstrating that bottom-up deficits in temporal encoding alter neural representations of speech in the presence of competing sounds. These findings will provide support for a link between noise-induced CS and impaired speech understanding in noise in listeners with NHTs and will provide valuable knowledge of the contributors to this communication impairment. This research will take place in an exceptional training environment and the PI will be mentored by two knowledgeable, accomplished scientists. These experiments will provide the PI with training in psychophysical task construction, brain activity recording, and signal processing methods and will prepare her well for a career as an independent scientist.